It is well known to form an image by phase contrast imaging methods in which phase modulation of light is converted into intensity modulation. As opposed to intensity modulation, phase modulation does not involve loss of energy.
In published patent application no. WO 96/34307, which is hereby incorporated by reference, a phase contrast imaging method is disclosed for calculating phasor values of a phase mask for synthesising a desired intensity pattern.
In “Securing Information with Optical Technologies”, Bahram Javidi, Physics Today, Vol. 50, No. 3, March 1997, pp 27-32, a method and system for optically securing information is proposed. An image is encrypted in a 4f-lens optical configuration (i.e. comprising two Fourier transforming lenses) by Fourier transforming the image and an input phase mask with a first lens of the 4f-lens system. A Fourier plane phase mask having phasor values eib(α,β) is positioned in the Fourier plane of the first lens and an encrypted image is formed with a second lens that Fourier transforms images in the Fourier plane of the first lens. The encrypted image is decrypted in a similar 4f-lens configuration in which another Fourier plane phase mask (the key) having phasor values e−ib(α,β) is positioned in the Fourier plane of the first lens.
It is a disadvantage of the known cryptographic method that encrypting an image both in the object plane and the Fourier plane leads to generation of speckle patterns in the decrypted image thereby corrupting data having been encrypted.
It is another disadvantage of the known cryptographic method that extremely accurate three-dimensional positioning of the phase mask in the Fourier plane is required for successful encryption and decryption.
It is yet another disadvantage of the known cryptographic method that both amplitude and phase have to be recorded in the encrypted mask.